Not doing a test-drive
Waiting until the day of the wedding to try on the whole ensemble is never a good idea. Whether he is buying or renting, a practice run, at least 24 hours before the wedding, will alert him to trousers that were hemmed too long, two left shoes or the simple fact that he really doesn't know how to tie that bow tie.
Forgetting the feet
Formalwear means formal shoes, and these are lightly constructed and almost always patent leather. When worn with the whole ensemble, they really do complete the look. (And while he's at it, have him pick up a pair of knee-length silk dress socks.)
It is impossible to take one's own measurements accurately. And sending in the size of a shirt, tuxedo or suit he already owns is another recipe for disaster, as brands differ in sizing.
The upside-down cummerbund
Here's the trick to remembering which way is right-side up: Gentlemen used to tuck their theater tickets into the pleats. To do that, the pleats have to be facing up. Pass on this tip to the groomsmen too!
Not worrying about quality
If he's buying a tux, he probably understands that it's worth shelling out a bit more dough for a suit that will last. But the same thinking should hold true for a rented tux—after all, he probably isn't the first one wearing it, and he definitely doesn't want that to be obvious. Same goes for the shoes—if the shop is throwing them in free, chances are they will look it.